Since November, at least 54 people have faced criminal complaints under the lèse-majesté law that prohibits any perceived insult to the Thai monarchy, according to the legal aid group Thai Lawyers for Human Rights. A number of them have been summoned by police, setting investigations in motion, the group said.

Most of them are protesters who have changed Thailand’s political landscape in recent months by defying long-held social taboos and openly questioning the crown’s role and influence. The palace has traditionally held an almost sacred status in Thai society. But many in the new protest movement see the crown as part of the royalist-military elite that, they say, is holding back democratic progress in their country rocked by frequent coups and political turmoil.

They have become more outspoken about the wealth, lifestyle and political clout of the king, who took the throne after his widely revered father died in 2016. Many have also questioned the legitimacy of the law that shields the monarchy from criticism.

Government spokesman Anucha Burapachaisri said authorities would enforce all applicable laws, including lèse-majesté.

“The government always welcomes all criticism, but in productive ways, not in ways that mislead, create misunderstanding or hate in society,” he said. “If any person doesn’t follow the rule of law or act correctly, they will be prosecuted.”

The lèse-majesté law has typically been used in times of political upheaval, particularly around military coups. Dozens of dissidents fled to Laos and Cambodia after a 2014 coup, when the law was used widely against critics of the military who were accused of undermining the crown. Some sought asylum in countries including France, where they say they still fear repercussions, even on another continent.

This week, a former civil servant was given the longest known sentence for the crime—more than 43 years in prison—after she was found guilty of uploading critical commentary to social-media platforms six years ago.

Anchan Preelert, 65 years old, was accused of uploading 29 audio clips to YouTube and

Facebook,

featuring the voice of a podcaster making remarks deemed insulting to the royal family. Ms. Anchan’s indictment record said she uploaded the content with the intention of causing damage to the monarchy and thus endangering national security.

Each charge against her was counted separately. Ms. Anchan was initially sentenced to 87 years in prison, but her punishment was reduced because she pleaded guilty to all charges, according to her lawyer. Her legal team said her request for bail while she prepared to appeal the verdict was denied.

Sunai Phasuk, a senior Thailand researcher for advocacy group Human Rights Watch, said the record sentence sent “a spine-chilling signal not only that critics of the monarchy won’t be tolerated, but they will also be punished to the extreme.”

Mr. Sunai said, “Thailand’s political tensions will now go from bad to worse.”

Thanathorn Juangroongruangkit is facing lèse-majesté investigation.

For months, protests have targeted the government of

Prayuth Chan-ocha,

a former coup leader who was elected prime minister in 2019. The mostly youth-led demonstrations have called for fresh elections. Some protesters have broadened their demands to include unprecedented curbs on the king’s power.

They have asked that his wealth be open to public auditing, that he relinquish control over two army units under his direct command and that royal defamation laws be abolished.

Among those facing a lèse-majesté investigation this week is Thanathorn Juangroongruangkit, the leader of an opposition party that a court dissolved last year over a financial-misconduct case he says was politically motivated. Thailand’s ministry of digital economy on Wednesday asked law enforcement to investigate a Facebook live video in which Mr. Thanathorn talked about the country’s plans to have Covid-19 vaccines manufactured by a company almost fully owned by the king.

Police said they had opened an investigation. Mr. Thanathorn said Thursday that he was innocent and that his remarks were intended to encourage transparent negotiations between the government and drugmakers.

Protesters have changed Thailand’s political landscape in recent months.

Political analysts say that using the lèse-majesté law widely could backfire.

“The whole thing is very much a gamble because the situation is already so volatile,” said James Buchanan, a visiting lecturer at Thailand’s Mahidol University International College. “They’re really adding fuel to the fire here.”

Mr. Anucha, the government spokesman, said that if authorities didn’t enforce the rule of law, they would be accused of being dysfunctional.

Write to Feliz Solomon at [email protected]